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A novel anisotropic conducting thin film having a conducting and insulating layered structure
67
A NOVEL ANISOTROPIC CONDUCTING THIN FILM HAVING A CONDUCTING AND INSULATING LAYERED STRUCTURE* TAKEO SHIMIDZU, TOMOKAZU IYODA, MASANORI ANDO, AKIRA OHTANI, TAKEHIRA KANEKO AND KENICHI HONDA Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606 (Japan)
(ReceivedJuly 26, 1987;acceptedSeptember15, 1987)
Electrochemical polymerization of Langmuir-Blodgett multilayers of amphiphilic pyrrole derivatives resulted in novel anisotropic conducting thin films (T. Iyoda, M. Ando, T. Kaneko, A. Ohtani, T. Shimidzu and K. Honda, Tetrahedron Lett., 27 (1986) 5633). They have an alternating layered structure of a conducting polypyrrole layer and an insulating alkyl chain layer. This paper deals with their syntheses, characterizations and functionalities.
1. SYNTHESESOF AMPHIPHILICPYRROLEMONOMERS Amphiphilic pyrrole derivatives such as 1 and 2 were synthesized according to the routes shown in the scheme of Fig. 1. ! was obtained by N-alkylation of 4methylpyrrole-3-carboxylic acid with stearyl bromide. 2 was formed by the reaction of stearyl crotonate with toluenesulphonylmethyl isocyanidel. 2. MONOLAYERSAND MULTILAYERSOF AMPHIPHILICPYRROLEMONOMERS Monolayers of the amphiphilic pyrrole monomer 1 or 2, or mixed monolayers of the monomer and octadecane 3, were spread from benzene solution. Octadecane, the spacer molecule, is added to the solution of the amphiphilic pyrrole in order to improve the packing of the alkyl chain moiety of amphiphiles in monolayers. Pure 1 and pure 2 did not form a stable and tightly packed monolayer, as shown in Fig. 2. In contrast, the n-A isotherms of the 1-3 mixed monolayer and those of 2 - 3 mixed monolayer suggested the formation of stable and tightly packed monolayers and were reproducible. Even on a neutral subphase, distinct and sharp increases in the surface pressure were observed and the maximum collapse pressure reached ca. 50 m N m - 1 (Fig. 2). Benzene was a better spreading solvent for amphiphiles than chloroform. The molar mixing ratios of 1:3 and 2:3 were varied from 1 to 4, and the mixed monolayer with a molar ratio of 2 was considered to give the closest packing * Paper presented at the Third International Conference on Langrauir-Blodgett Films, G6ttingen, F.R.G., July 26-31, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netbedands
68
T. SHIMIDZUe t
HO2C
CH3 1. K 2. CIBH37Br ,
H
H02C
CH3
THF
C18H37OH ÷ CH3CH:CHCOCI
H H
I C18H37
•
0.76(0.76) 0.12(0.11| 0.04(0.04|
C
~'~
al.
6(=-H) 6.45,7.29 1X-NHR 6(-CH2-1 L28
CH3CH:CHCO2CIsH37 H3C
CH3CH:CHCO2CmH37 + C H 3 - ~ S O 2 C H 2 N C :
CO2CI8H37
NaH . DME
H
~
!
C H H
0.76{0.76) 0,12(0.11) 0.04(0.041
6(=-a)
6.45,?.30 1.30
IH-NMR 6f-OH2-)
Fig. 1. Synthesesof amphiphilicpyrroles. from the n - A isotherm profile. This closest packing was proposed on the basis of the C P K model. The different limiting areas in the n - A isotherms of 1 and 2 would be explained by the different orienting angles between the plane of the pyrrole ring and the surface of the subphase. T h e stable mixed monolayers of 1 and 3 (2:1) or 2 and 3 (2:1) formed on a pure water subphase (10-22 °C) could be transferred onto the following hydrophobic substrates: silated quartz, silated indium-tin oxide (ITO)-deposited glass and ITOdeposited polyester ( I T O - P E ) (CELEC-K EC, Daicel Chemical Industries Ltd.; the sheet resistance of the ITO was 100-500f~/Z3). Silation of quartz and ITOdeposited glass was carried out by treating substrates in a toluene solution of trimethylchlorosilane (10~). The transfer ratio was 0.9-1.1, and more than 600 layers could be transferred as Y-type films. Y-type transfer was evidenced by the transfer ratio and the structural analyses by X-ray diffraction (XRD) and sectional transmission electron microscopy (TEM). The structure of the Langmuir-Blodgett (LB) films was examined by XRD experiment and small angle X-ray scattering (SAXS). The (00/) diffraction patterns were investigated. The range of 20 was c a . 4 °- 15 ° for XRD and ca. 00-5 ° for SAXS. The bilayer spacings d of the LB films were calculated to be as follows: 57 ~ for the 2-3 LB film on silated ITO-deposited glass transferred at 30 m N m - 1, 64/~ for the 2-3 LB film on I T O - P E transferred at 35 m N m - 1, and 65/~ for the 1-3 LB film on I T O - P E transferred at 35 mN m - t. It was shown that the bilayer spacings d varied according to the different substrates and the different surface pressures. This fact
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
69
2 T
E
5O
1 -
±
=2
H02C
CHj
(D t~
I
CleH]7
m 30 u) H3C
k_
n, 20 U
C02C~eH37
H
L
0 0
i
i
10
20
30
~0
50
Area per molecule / ~2 Fig. 2. Surface pressure-area isotherms for 1,2 and their mixed monolayers with 3 (subphase, 1 mmol 1-1 KH2PO4-Na2HPO4; pH 6.85-6.95; 17 °C).
shows that the tilting angles of the long axes of the film-forming molecules in LB films can be controlled by choosing the substrates and the surface pressures. When the surface pressure was too high, the ordered structure o1 the LB multilayer collapsed in a few days probably because of residual strain, even though the initial structure transferred was highly ordered. The maximum surface pressure for transfer which did not lead to collapse of the LB multilayer structure was c a . 30 mN m - ~ for the 1-3 LB multilayer and c a . 40 mN m - t for the 2-3 LB multilayer. More direct observation of the layered structure in cross-section of the LB film was attempted by T E M (mentioned later). 3. ELECTROCHEMICAL POLYMERIZATION OF THE MONOMERIC PYRROLE LANGMUIR-BLODGETT FILM
Amphiphilic pyrrole LB films for electropolymerization were constructed as Ytype films on either a silated ITO-deposited glass or I T O - P E . The resistance of the ITO layers of the substrates was low enough for their use as an anode for the electropolymerization of pyrrole. The main electrolyte solution used was LiC104-CH 3CN (0.05-0.1 mol 1-1). The electropolymerization of the LB film was carried out under conditions in which only the lower tip (a few millimetres) of the monomeric LB film was dipped in the electrolyte solution (Fig. 3). The region of the monomeric LB film under the liquid surface scarcely polymerized and dissolved in the solution. The polymerization of the LB film began near the liquid surface and then proceeded upwards (Fig. 4). The polymerization was observed by the change in the colour of the LB film (from the semitransparent white of the monomeric LB film to the blackish green or reddish
70
T. SHIMIDZUet al.
Fig. 3. Set-upfor the dectropolymer~tion of the LB film.
m i
Fig. 4. Appearanceof the LB filmduring electropolymerization. brown of the polymerized film). The coloured area of the LB film was spectroscopically confirmed to be polypyrrole (PPy) as follows. A visible-near-IR absorption spectrum of the electro-oxidized LB film is shown in Fig. 5. An absorption band near 500 nm was assigned to n - n * transition of PPy, and a broad absorption band near 1500 nm was assigned to the transition from the impurity level to the conduction band of PPy. These two absorption bands are characteristic of an ordinary doped PPy. An attenuated total reflection IR spectrum of the monomeric pyrrole LB film and that after electro-oxidation are shown in Fig. 6. The absorption band at 780 cm-1, assigned to C - - H out-of-plane bending at the 2 and 5 positions in the pyrrole ring, disappeared after electro-oxidation of the multilayer. These spectroscopic observations strongly suggested that the pyrrole moiety in the monomeric LB multilayer was electropolymerized. Small drops were observed only on the polymerized area. These drops were speculated to be the electrolyte solution which permeated into the LB film. The voltage for initiating the polymerization of the LB film was ca. 1 V. The polymerization speed was 2-4 mm h - 1 parallel to the film plane, and the boundary between the monomeric region and the polymerized region was clear.
71
ELECTROCHEMICAL POLYMERIZATIONOF LB FILMS
1.2
] ~
ele~dized
2 mixedwith 3_
1.0
0.8
C (:3
~ 0.6
H]C CO,Ct.H3;,
r'~
0./.
H 3_
C,eH~B
0.2 0
i
t
500
1000
I
I
1500 Wovelength/ n m
2000
2500
Fig. 5. Visible-near-IR absorption spectrum of the 2-3 LB film after electro-oxidation.
~
~
t/I
2 HC j CO~CLsH~I
C 13
,o
I--
4800
electrolyticollyo x i d i ~ mixedwith _3 T
3000
T
I
1800 1000 Wovenumber/ crn-~
~,
I
600
Fig. 6. Attenuated total reflectionIR spectra of the 2-3 LB multilayer and of the multilayer after electrooxidation. T h e electropolymerization of the LB film was also a t t e m p t e d in L i B F 4 - C H 3 C N , L i C I O 4 - - C H 3 C O O H a n d L i C 1 0 4 - H 2 0 . I n the case of L i B F 4 - C H 3 C N , the p o l y m e r i z a t i o n of the LB film was almost the same as that in the case of L i C 1 O a - C H 3 C N . I n the case of L i C I O 4 - C H 3 C O O H , the b o u n d a r y
72
T. SHIMIDZUe t
al.
between the monomeric area and the polymerized area.was less clear and the reproducibility of the polymerization speed was poorer. In contrast, when L i C I O 4 - H 2 0 was used as the electrolyte solution electropolymerization did not occur, probably because water could not permeate into the monomeric LB film whose outermost layer was hydrophobic and water was electrolysed at a lower potential than the initial potential for the polymerization of the amphiphilic pyrrole monomers in the LB film. 4.
D . C . C O N D U C T I V I T Y OF T H E P O L Y P Y R R O L E L A N G M U I R - - B L O D G E T T F I L M
As the substrate of the PPy LB film was a semiconductor, the PPy LB film had to be separated from the substrate for the measurement of the electrical conductivity parallel to the layer plane. The PPy LB film could be separated by an adhesive tape from the substrate because the PPy LB film had a greater mechanical strength than the monomeric LB film. The PPy LB film (200 layers) showed a highly anisotropic d.c. conductivity of ca. 10 orders of magnitude (aqt= 1 0 - 1 S c m -1, tr± = 1 0 - x l S cm-1) according to the preliminary measurements (Fig. 7). The d.c. conductivity atl parallel to the multilayer was measured by a four-electrode method. The d.c. conductivity tr± in the perpendicular direction was measured by putting an LB film in ohmic contact between two ITO electrodes (area, ca. 1 cm2).
//
(200
layers)
J
/I ,,
(
o,1 : l O - i S
)
•cm -I
Fig. 7. Anisotropicconductivityof the polymerized2-3 LB multilayer. 5. S T R U C T U R E S OF T H E M O N O M E R I C A N D T H E P O L Y M E R I Z E D L A N G M U I R - - B L O D G E T T FILMS
The structures of the monomeric LB film and the polymerized LB film were studied by XRD analysis and TEM2 of cross-sections of the LB film. Sectional T E M observations were made to confirm the layered structure shown by the XRD analysis. The electron microscopy observations were indispensable in evaluating the dimensions of the ordered domain. Although an electron micrograph of an imperfect cross-section of the multifold ridge which formed at "slow collapse" as a perturbed region has been reported 3, there have been no successful direct observations of the structure of the cross-section with an electron microscope. Here, the layered structure in the cross-section of the conducting PPy LB film was observed directly with a transmission electron microscope.
73
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
5.1. X-ray diffractive analysis The XRD pattern of the electropolymerized 2-3 multilayer (204 layers) on a silated ITO-deposited glass showed a sharp diffraction peak at 20 = 6.07 °, which was similar to that before electropolymerization (20 = 6.22 °) (Fig. 8). The monomeric LB film was prepared at a surface pressure of 30 mN m - ~ and a dipping speed of 50 mm min- 1. The multilayers before and after electropolymerization had bilayer spacings d of 56.7 ~ and 58.1/~ respectively, as was expected from their layered structures. If it is assumed that the observed peaks in the XRD patterns could be assigned to fourth-order diffraction, the resulting lattice constants are for the bilayer spacing d of Y-type LB film. The widths of the XRD peaks before and after electropolymerization had the same magnitude. This showed that the crystal boundaries in the LB film were not changed by electropolymerization. C02C,BH:~7
H]C
L~
~A
/
\
_2 mixed wilh 2
P
electrolyticully oxidized 2_ mixed with i
4
5
i
i
6
7
3_
20 1 ° Fig. 8. × R D patterns ofLB multilayers on si]atedITO-deDosited glass (204 layers).
The XRD pattern of the 2-3 multilayer (120 layers) on I T O - P E before and after electropolymerization showed sharp diffraction peaks (Fig. 9). The monomeric LB multilayer was prepared at a surface pressure of 35 mN m - 1 and a dipping speed of 50 mm min- ~. The diffraction peaks were assigned as shown in Fig. 9. The bilayer spacing d values were 64~, for the monomeric multilayer and 66/~ for the polymerized multilayer (Table I). The width of each peak was smaller than that of the corresponding peak when the silated ITO-deposited glass substrate was used. This reveals the larger size of the crystalline domain in the LB film on ITO-PE. This is speculated to result mainly from the higher surface pressure and the higher affinity between a monolayer and a substrate. The observed change in strength of each XRD peak due to polymerization was speculated to result from the change in the atomic scattering factor of the LB film caused by the doping of C104- into the PPy layer. These bilayer spacing d values of the multilayer on I T O - P E were greater than those of the multilayer on silated ITO-deposited glass by ca. 8/~. The difference
T. SHIMIDZU et al.
74
H3C C02CleH37 (004)
H 3_
C,8H~6
(006) In
n
(005) electrolyticoUy oxidized 2 [
mixed wilh 3_
/
(004)
4
/
/
/
\
i
I
t
I
5
6
7
8
29 / °
Fig. 9. XRD patterns ofLB multilayers on ITO-PE (120layers). TABLE I BILAYER SPACINGS
Sub str ate
Silated ITOdeposited glass ITO-PE
d OFTHE 2 - 3
LANGMUIR--BLODGETT MULTILAYERS
Method
Surface pressure
Bila yer spacing d (1~)
(mN m- 1)
Monomeric LB film
Polymerized l_Bfilm
30
57
58
XRD
30 35
50 64
66
SAXS XRD
was considered to result from the different substrates and the different surface pressures. The SAXS experiment was carried out only on the monomeric multilayer. The monomeric LB multilayer was prepared at a surface pressure of 30 m N m - 1 and a dipping speed of 5 0 m m m i n - L The calculated bilayer spacing d of the LB multilayer before polymerization was 50/L The slight increase in the bilayer spacings d caused by polymerization observed in the XRD experiment was speculated to be due to a change in the tilting angles of the film-forming molecules in the multilayers as a result of polymerization and doping with C104-. The tilting angles were calculated as follows: 78 ° for the 2-3
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
75
multilayer on silated ITO-deposited glass transferred at 30 m N m - 1, 60 ° for the 2-3 multilayer on I T O - P E transferred at 30 m N m - 1, and 90 ° for the 2-3 multilayer on I T O - P E transferred at 35 m N m - 1. 5.2. Sectional transmission electron microscopy observation
The layered structures in the cross-sections of the monomeric 2 - 3 LB film on I T O - P E transferred at 35 m N m - 1 and the polymerized film were directly observed with a JEM-1200EX transmission electron microscope (JEOL Ltd.). The method of specimen preparation was as follows. The monomefic or polymerized LB film on I T O - P E was treated with RuO4 vapour for 30min. The stained LB film was embedded in epoxy resin (60 °C, 12 h). The embedded LB film together with the I T O - P E was sectioned perpendicular to the film plane with a diamond cutter. The sliced pieces were 400-500 ,~ in thickness. The direct magnification was 100 000 x and the accelerating voltage was 120 kV. Figures 10 and 11 show enlarged electron micrographs of the cross-sections of the monomeric LB film and of the polymerized LB film respectively. All sections studied exhibited similar electron micrographs. The dark regions were considered to correspond to pyrrole or PPy units reacted with RuO4, while the light regions correspond to the alkyl chains. The striped pattern of dark and light lines thus demonstrates the alternating layered structure of the films. The spacing of the stripes shown in Fig. 10 corresponds to a bilayer spacing d of ca. 50 ~. This value is smaller than that obtained from the XRD pattern (64 ~) by ca. 20~. This difference was speculated to be mainly due to distortion in the sectioning process and the embedding process at 60 °C, which was higher than the melting point of the filmforming molecules of the monomeric LB film. The spacing of the stripes shown in Fig. 11 corresponds to a bilayer spacing d of 55-62 ~, a value close to that obtained from the XRD measurements. It is considered that the increased melting point and the increased mechanical strength of the polymerized LB film allows us to obtain undistorted and clear images of the film structure at the molecular level. While the layered structure is observed over the entire depth of the sample, some dislocations are visible. These dislocations might be due to artifacts resulting from the embedding and/or sectioning processes. (The distortion made the T E M picture ambiguous
Fig. 10. Sectionaltransmissionelectronmicrographof the 2-3 LB multilayer.
76
T. SHIMIDZUet al.
Fig. 11. Sectional transmission electron micrograph of the electropolymerized2-3 LB multilayer.
because the thickness of the specimen was 400-500 ,~, which was much larger than the observed bilayer spacing d.) 6. MECHANISM OF THE ELECTROPOLYMERIZATION OF THE PYRROLE LANGMUIR-BLODGETT FILM
The electropolymerization of LB films is unprecedented before the present work. Photochemical and thermal polymerization of LB films has been widely investigated, for example, on vinyl derivatives4, diacetylene derivatives s, butadiene derivatives6 and amino acid derivatives6. These photochemical and thermal polymerizations of the LB films take place topochemically in the solid state. The present electropolymerization also took place topochemically. The electropolymerization of the LB film proceeded at the contacting pyrrole derivatives in front of the electropolymerized pyrrole derivative, as shown in Fig. 12. It went in one direction, and a clear boundary between the polymerized area and the monomeric area was observed. We present the following polymerization mechanism for the pyrrole LB film, as shown in Fig. 12. The stages of electropolymerization are as follows.
77
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
I
11 I / / Z_.Y_./_../ V UCI0,-CH~CN" / A)
Fig. 12. Electrolytic polymerization mechanism of the LB film.
(1) The pyrrole derivative of the LB film below the liquid surface dissolved partially in acetonitrile but near the liquid surface it was disordered and was able to take various conformations freely and to contact the anode directly sufficiently to form the amorphous PPy. The amorphous PPy grew on the anode near the liquid surface so as to form a conducting bridge between the electrode and the ordered region of the pyrrole LB film. (2) The pyrrole moiety in the pyrrole LB film polymerized as the electrolyte solution permeated into the LB film because the conducting PPy formed functions as the electrode. The permeation of the electrolyte solution into the LB film probably proceeded by capillary attraction and the effect of the formation of an electrical double layer. It is considered that the difference between the redox potentials of a supporting electrolyte and the pyrrole derivative governs the polymerization, and only electronic conductivity plays the role in the polymerization to give the potential at the front. From the fact that the wet region appeared to coincide with the polymerized region, topochemical polymerization is suggested. In this stage of polymerization in the ordered LB film, the electron transfer is considered to take place in the layer plane as a result of current flow in the PPy LB layers. As the eliminated protons are considered to move easily in the wet region in the LB film, polymerization would be carried out smoothly. In order to examine the polymerization mechanism described above the following two experiments were carried out with systems avoiding electron transfer across the LB film perpendicular to the film plane. In the first experiment, a gold layer was deposited on part of the multilayered monomeric LB film on a silated quartz substrate as shown in Fig. 13. The electropolymerization of this LB film was attempted. The electropolymerization of the LB film proceeded not only in the gold-deposited area but also in the other area on which gold was not deposited. This shows that the LB film polymerized by electron transfer through the P P y layer parallel to the layer plane. In the second experiment, the organic insulating layer was used in order to inhibit electron transfer across the LB film perpendicular to the layer plane. Firstly,
78
T. SHIMIDZUe t al.
an insulating methyl stearate LB multilayer (70 layers, ca, .1700/~ in thickness) was set up as a Y-type film onto I T O - P E . Then a pyrrole LB multilayer (1-3) (88 layers, ca. 2600/~ in thickness) was set up on it (Fig. 14). The heterogeneous LB film structure was confirmed by XRD. The electropolymerization of this LB film was attempted by the following procedure.
usra
Pyrro~emonome~~ Pt wire - ~
~
~
J J----~'tr----~
]
~//< ~< ~<~//3 Fig. 13. quartz.
Set~up~rthee~ectr~p~ymeriza~n~ftheg~d~dep~sitedm~n~mericpyrr~eLB~m~nsi~ated
~II
ca. 1700A) ~
pol ~ y r r o l e
,sa ~ay°r:~ l l
Substa:ate
p~~i~~ }"•
LB film
~or ~hOus polypyrrole
Fig. 14. Electropolymerization of the monomeric pyrrole LB multilayer on the methyl stearate LB multilayer.
ELECTROCHEMICALPOLYMERIZATIONOF LB FILMS
79
(1) The lower tip of the multilayered monomeric LB film was dipped in C H 3 C N containing amphiphilic pyrrole m o n o m e r and LiCIO4 and then, by the application of an oxidation potential a m o r p h o u s PPy, which would act as a conducting bridge connecting the anode to the pyrrole layers, was formed at the region of the LB film near the liquid surface. (2) The electropolymerization in the ordered region of the LB film was attempted in the same set-up as for the electropolymerization of the pyrrole LB film which did not contain methyl stearate. In this stage, the used electrolyte solution did not contain pyrrole monomer. As a result, the polymerization in the LB film proceeded in the same way as in the pyrrole LB film which did not contain such a thick insulating layer. This observation strongly supports the polymerization mechanism. In addition, the observed linear correlation between the absorbance in the near-IR region and the number of layers also suggested that all layers were polymerized. The topochemical electropolymerization of the LB film described above can be called "self-assisted electropolymerization". This type of polymerization also realizes an ideal contact to the conducting LB film at the molecular level. Such a unique electropolymerization mechanism of the LB film is worth investigation. ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid from the Ministry of Education of Japan. REFERENCES 1 A.M. van Leusen, H. Siderius, B. E. Hoogenboom and D. van Leusen, Tetrahedron Lett., (1972) 5337. 2 T. Iyoda, M. Ando, T. Kaneko, A. Ohtani, T. Shimidzu and K. Honda, Langmuir, in the press. 3 A. Barraud, J. Leloup, P. Maire and A. Ruaudel-Teixier, Thin Solid Films, 133 (1985) 133. 4 A. Barraud, Thin Solid Films, 99 (1983) 3 !7. 5 G. Lieser, B. Tieke and G. Wegner, Thin Solid Films, 68 (1980) 77. 6 K. Fukuda, Y. Shibasaki and H. Nakahara, Thin Solid Films, 133 (1985) 39.
A NOVEL ANISOTROPIC CONDUCTING THIN FILM HAVING A CONDUCTING AND INSULATING LAYERED STRUCTURE* TAKEO SHIMIDZU, TOMOKAZU IYODA, MASANORI ANDO, AKIRA OHTANI, TAKEHIRA KANEKO AND KENICHI HONDA Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606 (Japan)
(ReceivedJuly 26, 1987;acceptedSeptember15, 1987)
Electrochemical polymerization of Langmuir-Blodgett multilayers of amphiphilic pyrrole derivatives resulted in novel anisotropic conducting thin films (T. Iyoda, M. Ando, T. Kaneko, A. Ohtani, T. Shimidzu and K. Honda, Tetrahedron Lett., 27 (1986) 5633). They have an alternating layered structure of a conducting polypyrrole layer and an insulating alkyl chain layer. This paper deals with their syntheses, characterizations and functionalities.
1. SYNTHESESOF AMPHIPHILICPYRROLEMONOMERS Amphiphilic pyrrole derivatives such as 1 and 2 were synthesized according to the routes shown in the scheme of Fig. 1. ! was obtained by N-alkylation of 4methylpyrrole-3-carboxylic acid with stearyl bromide. 2 was formed by the reaction of stearyl crotonate with toluenesulphonylmethyl isocyanidel. 2. MONOLAYERSAND MULTILAYERSOF AMPHIPHILICPYRROLEMONOMERS Monolayers of the amphiphilic pyrrole monomer 1 or 2, or mixed monolayers of the monomer and octadecane 3, were spread from benzene solution. Octadecane, the spacer molecule, is added to the solution of the amphiphilic pyrrole in order to improve the packing of the alkyl chain moiety of amphiphiles in monolayers. Pure 1 and pure 2 did not form a stable and tightly packed monolayer, as shown in Fig. 2. In contrast, the n-A isotherms of the 1-3 mixed monolayer and those of 2 - 3 mixed monolayer suggested the formation of stable and tightly packed monolayers and were reproducible. Even on a neutral subphase, distinct and sharp increases in the surface pressure were observed and the maximum collapse pressure reached ca. 50 m N m - 1 (Fig. 2). Benzene was a better spreading solvent for amphiphiles than chloroform. The molar mixing ratios of 1:3 and 2:3 were varied from 1 to 4, and the mixed monolayer with a molar ratio of 2 was considered to give the closest packing * Paper presented at the Third International Conference on Langrauir-Blodgett Films, G6ttingen, F.R.G., July 26-31, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netbedands
68
T. SHIMIDZUe t
HO2C
CH3 1. K 2. CIBH37Br ,
H
H02C
CH3
THF
C18H37OH ÷ CH3CH:CHCOCI
H H
I C18H37
•
0.76(0.76) 0.12(0.11| 0.04(0.04|
C
~'~
al.
6(=-H) 6.45,7.29 1X-NHR 6(-CH2-1 L28
CH3CH:CHCO2CIsH37 H3C
CH3CH:CHCO2CmH37 + C H 3 - ~ S O 2 C H 2 N C :
CO2CI8H37
NaH . DME
H
~
!
C H H
0.76{0.76) 0,12(0.11) 0.04(0.041
6(=-a)
6.45,?.30 1.30
IH-NMR 6f-OH2-)
Fig. 1. Synthesesof amphiphilicpyrroles. from the n - A isotherm profile. This closest packing was proposed on the basis of the C P K model. The different limiting areas in the n - A isotherms of 1 and 2 would be explained by the different orienting angles between the plane of the pyrrole ring and the surface of the subphase. T h e stable mixed monolayers of 1 and 3 (2:1) or 2 and 3 (2:1) formed on a pure water subphase (10-22 °C) could be transferred onto the following hydrophobic substrates: silated quartz, silated indium-tin oxide (ITO)-deposited glass and ITOdeposited polyester ( I T O - P E ) (CELEC-K EC, Daicel Chemical Industries Ltd.; the sheet resistance of the ITO was 100-500f~/Z3). Silation of quartz and ITOdeposited glass was carried out by treating substrates in a toluene solution of trimethylchlorosilane (10~). The transfer ratio was 0.9-1.1, and more than 600 layers could be transferred as Y-type films. Y-type transfer was evidenced by the transfer ratio and the structural analyses by X-ray diffraction (XRD) and sectional transmission electron microscopy (TEM). The structure of the Langmuir-Blodgett (LB) films was examined by XRD experiment and small angle X-ray scattering (SAXS). The (00/) diffraction patterns were investigated. The range of 20 was c a . 4 °- 15 ° for XRD and ca. 00-5 ° for SAXS. The bilayer spacings d of the LB films were calculated to be as follows: 57 ~ for the 2-3 LB film on silated ITO-deposited glass transferred at 30 m N m - 1, 64/~ for the 2-3 LB film on I T O - P E transferred at 35 m N m - 1, and 65/~ for the 1-3 LB film on I T O - P E transferred at 35 mN m - t. It was shown that the bilayer spacings d varied according to the different substrates and the different surface pressures. This fact
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
69
2 T
E
5O
1 -
±
=2
H02C
CHj
(D t~
I
CleH]7
m 30 u) H3C
k_
n, 20 U
C02C~eH37
H
L
0 0
i
i
10
20
30
~0
50
Area per molecule / ~2 Fig. 2. Surface pressure-area isotherms for 1,2 and their mixed monolayers with 3 (subphase, 1 mmol 1-1 KH2PO4-Na2HPO4; pH 6.85-6.95; 17 °C).
shows that the tilting angles of the long axes of the film-forming molecules in LB films can be controlled by choosing the substrates and the surface pressures. When the surface pressure was too high, the ordered structure o1 the LB multilayer collapsed in a few days probably because of residual strain, even though the initial structure transferred was highly ordered. The maximum surface pressure for transfer which did not lead to collapse of the LB multilayer structure was c a . 30 mN m - ~ for the 1-3 LB multilayer and c a . 40 mN m - t for the 2-3 LB multilayer. More direct observation of the layered structure in cross-section of the LB film was attempted by T E M (mentioned later). 3. ELECTROCHEMICAL POLYMERIZATION OF THE MONOMERIC PYRROLE LANGMUIR-BLODGETT FILM
Amphiphilic pyrrole LB films for electropolymerization were constructed as Ytype films on either a silated ITO-deposited glass or I T O - P E . The resistance of the ITO layers of the substrates was low enough for their use as an anode for the electropolymerization of pyrrole. The main electrolyte solution used was LiC104-CH 3CN (0.05-0.1 mol 1-1). The electropolymerization of the LB film was carried out under conditions in which only the lower tip (a few millimetres) of the monomeric LB film was dipped in the electrolyte solution (Fig. 3). The region of the monomeric LB film under the liquid surface scarcely polymerized and dissolved in the solution. The polymerization of the LB film began near the liquid surface and then proceeded upwards (Fig. 4). The polymerization was observed by the change in the colour of the LB film (from the semitransparent white of the monomeric LB film to the blackish green or reddish
70
T. SHIMIDZUet al.
Fig. 3. Set-upfor the dectropolymer~tion of the LB film.
m i
Fig. 4. Appearanceof the LB filmduring electropolymerization. brown of the polymerized film). The coloured area of the LB film was spectroscopically confirmed to be polypyrrole (PPy) as follows. A visible-near-IR absorption spectrum of the electro-oxidized LB film is shown in Fig. 5. An absorption band near 500 nm was assigned to n - n * transition of PPy, and a broad absorption band near 1500 nm was assigned to the transition from the impurity level to the conduction band of PPy. These two absorption bands are characteristic of an ordinary doped PPy. An attenuated total reflection IR spectrum of the monomeric pyrrole LB film and that after electro-oxidation are shown in Fig. 6. The absorption band at 780 cm-1, assigned to C - - H out-of-plane bending at the 2 and 5 positions in the pyrrole ring, disappeared after electro-oxidation of the multilayer. These spectroscopic observations strongly suggested that the pyrrole moiety in the monomeric LB multilayer was electropolymerized. Small drops were observed only on the polymerized area. These drops were speculated to be the electrolyte solution which permeated into the LB film. The voltage for initiating the polymerization of the LB film was ca. 1 V. The polymerization speed was 2-4 mm h - 1 parallel to the film plane, and the boundary between the monomeric region and the polymerized region was clear.
71
ELECTROCHEMICAL POLYMERIZATIONOF LB FILMS
1.2
] ~
ele~dized
2 mixedwith 3_
1.0
0.8
C (:3
~ 0.6
H]C CO,Ct.H3;,
r'~
0./.
H 3_
C,eH~B
0.2 0
i
t
500
1000
I
I
1500 Wovelength/ n m
2000
2500
Fig. 5. Visible-near-IR absorption spectrum of the 2-3 LB film after electro-oxidation.
~
~
t/I
2 HC j CO~CLsH~I
C 13
,o
I--
4800
electrolyticollyo x i d i ~ mixedwith _3 T
3000
T
I
1800 1000 Wovenumber/ crn-~
~,
I
600
Fig. 6. Attenuated total reflectionIR spectra of the 2-3 LB multilayer and of the multilayer after electrooxidation. T h e electropolymerization of the LB film was also a t t e m p t e d in L i B F 4 - C H 3 C N , L i C I O 4 - - C H 3 C O O H a n d L i C 1 0 4 - H 2 0 . I n the case of L i B F 4 - C H 3 C N , the p o l y m e r i z a t i o n of the LB film was almost the same as that in the case of L i C 1 O a - C H 3 C N . I n the case of L i C I O 4 - C H 3 C O O H , the b o u n d a r y
72
T. SHIMIDZUe t
al.
between the monomeric area and the polymerized area.was less clear and the reproducibility of the polymerization speed was poorer. In contrast, when L i C I O 4 - H 2 0 was used as the electrolyte solution electropolymerization did not occur, probably because water could not permeate into the monomeric LB film whose outermost layer was hydrophobic and water was electrolysed at a lower potential than the initial potential for the polymerization of the amphiphilic pyrrole monomers in the LB film. 4.
D . C . C O N D U C T I V I T Y OF T H E P O L Y P Y R R O L E L A N G M U I R - - B L O D G E T T F I L M
As the substrate of the PPy LB film was a semiconductor, the PPy LB film had to be separated from the substrate for the measurement of the electrical conductivity parallel to the layer plane. The PPy LB film could be separated by an adhesive tape from the substrate because the PPy LB film had a greater mechanical strength than the monomeric LB film. The PPy LB film (200 layers) showed a highly anisotropic d.c. conductivity of ca. 10 orders of magnitude (aqt= 1 0 - 1 S c m -1, tr± = 1 0 - x l S cm-1) according to the preliminary measurements (Fig. 7). The d.c. conductivity atl parallel to the multilayer was measured by a four-electrode method. The d.c. conductivity tr± in the perpendicular direction was measured by putting an LB film in ohmic contact between two ITO electrodes (area, ca. 1 cm2).
//
(200
layers)
J
/I ,,
(
o,1 : l O - i S
)
•cm -I
Fig. 7. Anisotropicconductivityof the polymerized2-3 LB multilayer. 5. S T R U C T U R E S OF T H E M O N O M E R I C A N D T H E P O L Y M E R I Z E D L A N G M U I R - - B L O D G E T T FILMS
The structures of the monomeric LB film and the polymerized LB film were studied by XRD analysis and TEM2 of cross-sections of the LB film. Sectional T E M observations were made to confirm the layered structure shown by the XRD analysis. The electron microscopy observations were indispensable in evaluating the dimensions of the ordered domain. Although an electron micrograph of an imperfect cross-section of the multifold ridge which formed at "slow collapse" as a perturbed region has been reported 3, there have been no successful direct observations of the structure of the cross-section with an electron microscope. Here, the layered structure in the cross-section of the conducting PPy LB film was observed directly with a transmission electron microscope.
73
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
5.1. X-ray diffractive analysis The XRD pattern of the electropolymerized 2-3 multilayer (204 layers) on a silated ITO-deposited glass showed a sharp diffraction peak at 20 = 6.07 °, which was similar to that before electropolymerization (20 = 6.22 °) (Fig. 8). The monomeric LB film was prepared at a surface pressure of 30 mN m - ~ and a dipping speed of 50 mm min- 1. The multilayers before and after electropolymerization had bilayer spacings d of 56.7 ~ and 58.1/~ respectively, as was expected from their layered structures. If it is assumed that the observed peaks in the XRD patterns could be assigned to fourth-order diffraction, the resulting lattice constants are for the bilayer spacing d of Y-type LB film. The widths of the XRD peaks before and after electropolymerization had the same magnitude. This showed that the crystal boundaries in the LB film were not changed by electropolymerization. C02C,BH:~7
H]C
L~
~A
/
\
_2 mixed wilh 2
P
electrolyticully oxidized 2_ mixed with i
4
5
i
i
6
7
3_
20 1 ° Fig. 8. × R D patterns ofLB multilayers on si]atedITO-deDosited glass (204 layers).
The XRD pattern of the 2-3 multilayer (120 layers) on I T O - P E before and after electropolymerization showed sharp diffraction peaks (Fig. 9). The monomeric LB multilayer was prepared at a surface pressure of 35 mN m - 1 and a dipping speed of 50 mm min- ~. The diffraction peaks were assigned as shown in Fig. 9. The bilayer spacing d values were 64~, for the monomeric multilayer and 66/~ for the polymerized multilayer (Table I). The width of each peak was smaller than that of the corresponding peak when the silated ITO-deposited glass substrate was used. This reveals the larger size of the crystalline domain in the LB film on ITO-PE. This is speculated to result mainly from the higher surface pressure and the higher affinity between a monolayer and a substrate. The observed change in strength of each XRD peak due to polymerization was speculated to result from the change in the atomic scattering factor of the LB film caused by the doping of C104- into the PPy layer. These bilayer spacing d values of the multilayer on I T O - P E were greater than those of the multilayer on silated ITO-deposited glass by ca. 8/~. The difference
T. SHIMIDZU et al.
74
H3C C02CleH37 (004)
H 3_
C,8H~6
(006) In
n
(005) electrolyticoUy oxidized 2 [
mixed wilh 3_
/
(004)
4
/
/
/
\
i
I
t
I
5
6
7
8
29 / °
Fig. 9. XRD patterns ofLB multilayers on ITO-PE (120layers). TABLE I BILAYER SPACINGS
Sub str ate
Silated ITOdeposited glass ITO-PE
d OFTHE 2 - 3
LANGMUIR--BLODGETT MULTILAYERS
Method
Surface pressure
Bila yer spacing d (1~)
(mN m- 1)
Monomeric LB film
Polymerized l_Bfilm
30
57
58
XRD
30 35
50 64
66
SAXS XRD
was considered to result from the different substrates and the different surface pressures. The SAXS experiment was carried out only on the monomeric multilayer. The monomeric LB multilayer was prepared at a surface pressure of 30 m N m - 1 and a dipping speed of 5 0 m m m i n - L The calculated bilayer spacing d of the LB multilayer before polymerization was 50/L The slight increase in the bilayer spacings d caused by polymerization observed in the XRD experiment was speculated to be due to a change in the tilting angles of the film-forming molecules in the multilayers as a result of polymerization and doping with C104-. The tilting angles were calculated as follows: 78 ° for the 2-3
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
75
multilayer on silated ITO-deposited glass transferred at 30 m N m - 1, 60 ° for the 2-3 multilayer on I T O - P E transferred at 30 m N m - 1, and 90 ° for the 2-3 multilayer on I T O - P E transferred at 35 m N m - 1. 5.2. Sectional transmission electron microscopy observation
The layered structures in the cross-sections of the monomeric 2 - 3 LB film on I T O - P E transferred at 35 m N m - 1 and the polymerized film were directly observed with a JEM-1200EX transmission electron microscope (JEOL Ltd.). The method of specimen preparation was as follows. The monomefic or polymerized LB film on I T O - P E was treated with RuO4 vapour for 30min. The stained LB film was embedded in epoxy resin (60 °C, 12 h). The embedded LB film together with the I T O - P E was sectioned perpendicular to the film plane with a diamond cutter. The sliced pieces were 400-500 ,~ in thickness. The direct magnification was 100 000 x and the accelerating voltage was 120 kV. Figures 10 and 11 show enlarged electron micrographs of the cross-sections of the monomeric LB film and of the polymerized LB film respectively. All sections studied exhibited similar electron micrographs. The dark regions were considered to correspond to pyrrole or PPy units reacted with RuO4, while the light regions correspond to the alkyl chains. The striped pattern of dark and light lines thus demonstrates the alternating layered structure of the films. The spacing of the stripes shown in Fig. 10 corresponds to a bilayer spacing d of ca. 50 ~. This value is smaller than that obtained from the XRD pattern (64 ~) by ca. 20~. This difference was speculated to be mainly due to distortion in the sectioning process and the embedding process at 60 °C, which was higher than the melting point of the filmforming molecules of the monomeric LB film. The spacing of the stripes shown in Fig. 11 corresponds to a bilayer spacing d of 55-62 ~, a value close to that obtained from the XRD measurements. It is considered that the increased melting point and the increased mechanical strength of the polymerized LB film allows us to obtain undistorted and clear images of the film structure at the molecular level. While the layered structure is observed over the entire depth of the sample, some dislocations are visible. These dislocations might be due to artifacts resulting from the embedding and/or sectioning processes. (The distortion made the T E M picture ambiguous
Fig. 10. Sectionaltransmissionelectronmicrographof the 2-3 LB multilayer.
76
T. SHIMIDZUet al.
Fig. 11. Sectional transmission electron micrograph of the electropolymerized2-3 LB multilayer.
because the thickness of the specimen was 400-500 ,~, which was much larger than the observed bilayer spacing d.) 6. MECHANISM OF THE ELECTROPOLYMERIZATION OF THE PYRROLE LANGMUIR-BLODGETT FILM
The electropolymerization of LB films is unprecedented before the present work. Photochemical and thermal polymerization of LB films has been widely investigated, for example, on vinyl derivatives4, diacetylene derivatives s, butadiene derivatives6 and amino acid derivatives6. These photochemical and thermal polymerizations of the LB films take place topochemically in the solid state. The present electropolymerization also took place topochemically. The electropolymerization of the LB film proceeded at the contacting pyrrole derivatives in front of the electropolymerized pyrrole derivative, as shown in Fig. 12. It went in one direction, and a clear boundary between the polymerized area and the monomeric area was observed. We present the following polymerization mechanism for the pyrrole LB film, as shown in Fig. 12. The stages of electropolymerization are as follows.
77
ELECTROCHEMICAL POLYMERIZATION OF LB FILMS
I
11 I / / Z_.Y_./_../ V UCI0,-CH~CN" / A)
Fig. 12. Electrolytic polymerization mechanism of the LB film.
(1) The pyrrole derivative of the LB film below the liquid surface dissolved partially in acetonitrile but near the liquid surface it was disordered and was able to take various conformations freely and to contact the anode directly sufficiently to form the amorphous PPy. The amorphous PPy grew on the anode near the liquid surface so as to form a conducting bridge between the electrode and the ordered region of the pyrrole LB film. (2) The pyrrole moiety in the pyrrole LB film polymerized as the electrolyte solution permeated into the LB film because the conducting PPy formed functions as the electrode. The permeation of the electrolyte solution into the LB film probably proceeded by capillary attraction and the effect of the formation of an electrical double layer. It is considered that the difference between the redox potentials of a supporting electrolyte and the pyrrole derivative governs the polymerization, and only electronic conductivity plays the role in the polymerization to give the potential at the front. From the fact that the wet region appeared to coincide with the polymerized region, topochemical polymerization is suggested. In this stage of polymerization in the ordered LB film, the electron transfer is considered to take place in the layer plane as a result of current flow in the PPy LB layers. As the eliminated protons are considered to move easily in the wet region in the LB film, polymerization would be carried out smoothly. In order to examine the polymerization mechanism described above the following two experiments were carried out with systems avoiding electron transfer across the LB film perpendicular to the film plane. In the first experiment, a gold layer was deposited on part of the multilayered monomeric LB film on a silated quartz substrate as shown in Fig. 13. The electropolymerization of this LB film was attempted. The electropolymerization of the LB film proceeded not only in the gold-deposited area but also in the other area on which gold was not deposited. This shows that the LB film polymerized by electron transfer through the P P y layer parallel to the layer plane. In the second experiment, the organic insulating layer was used in order to inhibit electron transfer across the LB film perpendicular to the layer plane. Firstly,
78
T. SHIMIDZUe t al.
an insulating methyl stearate LB multilayer (70 layers, ca, .1700/~ in thickness) was set up as a Y-type film onto I T O - P E . Then a pyrrole LB multilayer (1-3) (88 layers, ca. 2600/~ in thickness) was set up on it (Fig. 14). The heterogeneous LB film structure was confirmed by XRD. The electropolymerization of this LB film was attempted by the following procedure.
usra
Pyrro~emonome~~ Pt wire - ~
~
~
J J----~'tr----~
]
~//< ~< ~<~//3 Fig. 13. quartz.
Set~up~rthee~ectr~p~ymeriza~n~ftheg~d~dep~sitedm~n~mericpyrr~eLB~m~nsi~ated
~II
ca. 1700A) ~
pol ~ y r r o l e
,sa ~ay°r:~ l l
Substa:ate
p~~i~~ }"•
LB film
~or ~hOus polypyrrole
Fig. 14. Electropolymerization of the monomeric pyrrole LB multilayer on the methyl stearate LB multilayer.
ELECTROCHEMICALPOLYMERIZATIONOF LB FILMS
79
(1) The lower tip of the multilayered monomeric LB film was dipped in C H 3 C N containing amphiphilic pyrrole m o n o m e r and LiCIO4 and then, by the application of an oxidation potential a m o r p h o u s PPy, which would act as a conducting bridge connecting the anode to the pyrrole layers, was formed at the region of the LB film near the liquid surface. (2) The electropolymerization in the ordered region of the LB film was attempted in the same set-up as for the electropolymerization of the pyrrole LB film which did not contain methyl stearate. In this stage, the used electrolyte solution did not contain pyrrole monomer. As a result, the polymerization in the LB film proceeded in the same way as in the pyrrole LB film which did not contain such a thick insulating layer. This observation strongly supports the polymerization mechanism. In addition, the observed linear correlation between the absorbance in the near-IR region and the number of layers also suggested that all layers were polymerized. The topochemical electropolymerization of the LB film described above can be called "self-assisted electropolymerization". This type of polymerization also realizes an ideal contact to the conducting LB film at the molecular level. Such a unique electropolymerization mechanism of the LB film is worth investigation. ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid from the Ministry of Education of Japan. REFERENCES 1 A.M. van Leusen, H. Siderius, B. E. Hoogenboom and D. van Leusen, Tetrahedron Lett., (1972) 5337. 2 T. Iyoda, M. Ando, T. Kaneko, A. Ohtani, T. Shimidzu and K. Honda, Langmuir, in the press. 3 A. Barraud, J. Leloup, P. Maire and A. Ruaudel-Teixier, Thin Solid Films, 133 (1985) 133. 4 A. Barraud, Thin Solid Films, 99 (1983) 3 !7. 5 G. Lieser, B. Tieke and G. Wegner, Thin Solid Films, 68 (1980) 77. 6 K. Fukuda, Y. Shibasaki and H. Nakahara, Thin Solid Films, 133 (1985) 39.